The present invention relates to a solid solution that comprises a complex between a photoluminescent polymer and a transition metal capable of binding to an alkene. Such a solid solution can be employed as a sensor for monitoring the level of at least one gaseous alkene (also known as an ‘olefin’). The invention finds particular utility in monitoring gaseous ethylene during shipment and storage of fresh produce. A robust market exists for the sale of ethylene detection and ethylene removal products, but these products have various shortcomings, noted below. New strategies for sensing ethylene at commercially relevant levels are essential to development of cost-effective monitoring devices.
Three environmental factors—temperature, humidity and ethylene concentration—influence the development and ripening of fresh produce. Control over these three factors is essential for efficient produce transport, storage and induced ripening. Temperature and humidity are easily and precisely controlled, while ethylene concentration is more difficult to control. It is commercially important to avoid premature exposure of fruits, vegetables, flowers and other plant products to ethylene because exposure to as little as 10 ppb ethylene initiates ripening. Moreover, while short-term exposure to high ethylene concentrations (1000–100,000 ppb) is required to induce rapid ripening, prolonged exposure inevitably leads to premature rotting and attendant financial losses. On the other hand, timely exposure of unripe fruit to ethylene at a suitable level desirably induces ripening.
Commercial fruit and vegetable growers harvest products in a slightly immature state to facilitate processing and transport, and store the unripe fruit in carefully controlled environments until ripening is desired. To initiate ripening, produce is processed to ensure that the product reaches market in an optimal state. Each product type requires specific conditions, including a specific ethylene concentration range and duration of exposure, for optimal market quality. Suitable ethylene control involves both (1) monitoring ethylene gas concentration over a range of between about 10 and 100,000 ppb and (2) adjusting the concentration as needed.
Ethylene is typically measured by metal oxide resistivity, by chemiluminescence on reaction with ozone, or by gas chromatography. Each method has drawbacks. Metal oxide sensors can be inaccurate as they are susceptible to interference from other analytes, from flow rate variations and from ambient oxygen. Chemiluminescence systems require ozone generation and are therefore expensive and potentially dangerous. Gas chromatography is accurate and specific but the instrumentation is costly and the method is not well-suited to real time sensing. Moreover, existing ethylene sensors are not ideal for routine use because they lack sufficient sensitivity and they are prohibitively expensive. Available commercial sensors cannot function below 200 ppb ethylene, notwithstanding the botanically-relevant 10 ppb level. The $1,000–$13,000 cost per unit also precludes widespread use of these devices.
Likewise, existing ethylene removal strategies are hampered by expense and by the shortcomings in available sensors. Existing removal strategies include simple adsorption and oxidation by supported permanganate. A catalytic oxidation device, the Bio-KES 348 Ethylene Removal System, can maintain ethylene levels as low as 10 ppb; but requires continuous UV irradiation and consumes 475 Watts. If this device were coupled to a low cost, accurate sensor/actuator system, it could be operated cost effectively in ripening chambers.
Some metals are well known to interact strongly with alkenes under some conditions, and are therefore a reasonable choice for sensor handles. There is precedent for the use of, e.g., coinage metals in ethylene sensing, as plants use copper in their ethylene receptor proteins and silver is used both to inhibit the ethylene response in cut flowers and to extract alkenes from petrochemical feedstreams. The chemistry of coinage metals provides fertile ground for the design of new ethylene sensor technology.
Coinage metals are used to separate gaseous alkenes and alkanes (also called ‘paraffins’) in the petrochemical industry. In solution separations, silver(I) or copper(I) salts are dissolved in a stabilizing solvent, thereby promoting metal-alkene interaction while inhibiting decomposition of the active metal ions. When crude gas streams are passed through the solution, alkenes bind to metal ions therein, and an alkene-enriched solution is collected. The alkenes are released from solution by heating, to produce a high purity alkene gas mixture.
Gaseous alkenes and alkanes are also separated from input gas streams by differential membrane permeability in facilitated transport processes wherein silver(I) or copper(I) salts are suspended in a polymer membrane that acts both as a support and as a metal ion ligand. The metal ions preferentially bind alkenes from the input gas stream, and the alkenes pass through the membrane by rapid diffusion from one metal center to another. The metal ions selectively facilitate the passage of alkenes through the membrane, thereby generating a purified alkene gas stream. A schematic diagram of facilitated transport is shown in FIG. 1. Facilitated transport is distinguished from the present invention in that it does not involve luminescence or sensing, and cannot because the polymers there employed lack the extended conjugation structures employed in the present invention.
Silver(I) salts are highly effective in facilitated transport applications because membranes impregnated with Ag(I) salts are stable and bind alkenes strongly but reversibly. Bound alkenes are readily removed from the membranes by mild desorption methods such as heating. Cu(I) can also be used in facilitated transport processes, but care must be taken to minimize disproportionation of the Cu(I) ion. Since disproportionation is bimolecular, Cu(I)-impregnated polymers are stable only if the polymer separates the metal centers effectively.
A key criterion for an effective transport polymer appears to be that the polymer bears a functional group capable of interacting with the metal centers. Polyoxazoline, cellulose acetate, polypyrrolidinone and polyethyleneoxide polymers can effectively separate alkenes and alkanes in commercial facilitated transport membranes. Many of these polymers contain electronegative oxygen atoms to bind the metal ions. Also, interaction between silver(I) ions and the oxygen atoms promotes the alkene-Ag(I) interaction. Ethylene transport by poly(vinyl methyl ketone) (PVMK) has been studied in some detail, revealing the molecular interactions that occur in this process. Silver ions interact directly with the carbonyl groups in PVMK, decreasing the frequency of the carbonyl vibrations. When ethylene binds to the silver ions, the carbonyl vibrational frequency increases slightly, suggesting that both the carbonyl and the alkene are within the coordination sphere of the metal. The fact that spectral changes are observed demonstrates that the polymer responds to the presence of the alkene. Nevertheless, there has been no suggestion to employ such polymers in a sensor system.
The efficacy of facilitated transport membranes depends on easy access of the alkene to the metal centers in the polymer matrix. A primary determinant of access to the metal ion is the strength of interaction between the metal cation and the counter-anion. Considerable improvement in the rate of alkene transport is achievable by varying the counter-ion. Because ethylene binding to closed shell Ag+ and Cu+ ions is calculated to involve both covalent and electrostatic components, soft electron-rich counter-ions compete with alkenes more effectively than hard electron-poor ions.
Counter-ion size and polarizability are both important to matrix structure. Large bulky counter-ions cannot approach the metal ion closely, thereby decreasing the strength of the electrostatic attraction. At the same time, small alkene molecules can enter spaces created in the polymer matrix by such large counter-ions. In 1-hexene:Ag(I) complexes with CF3CO2−, BF4−, NO3−, and CF3SO3− counter-ions, the metal-alkene bond strength correlates more strongly with the size of the counter-ion than with its basicity. On the other hand, the donor ability of the ion is also a matrix structure determinant. For example, poly(2-ethyl-2-oxazoline) impregnated with AgBF4 gives rise to a more porous polymer matrix and to substantially more rapid passage of alkene than the same polymer impregnated with AgClO4. In the aforementioned system, the silver ions are three-coordinate in the presence of the electron poor BF4− anion and four-coordinate in the presence of ClO4−, an observation that is more reasonably explained by donor ability than by size. Interestingly, there has been little effort to apply many of the very large, weakly interacting counter-anions commonly used in inorganic synthesis to facilitated transport systems.